The invention relates to a method for enhanced processing of signals received from Fabry-Perot sensors. Fabry-Perot sensors have broad utility for applications which require monitoring of absolute, static displacements and small, dynamic vibrations or oscillating changes. For example, their simplicity of design allows these sensors to be embedded into industrial applications, including gas turbines, engines, pressure vessels, pipelines, buildings or other structures, in order to provide information about pressure, temperature, strain, vibration, or acceleration within the structure. Their size, durability, and fast response time make these sensors advantageous. Examples of such sensors or arrangements incorporating such sensors have been developed by the inventor and/or assignee of this application include copending U.S. patent application Ser. No. 11/048,521, which published as U.S. Patent Publication 2005/0244096; Ser. No. 11/105,651, which published as U.S. Patent Publication 2005/0231729; and Ser. No. 11/106,750, which published as U.S. Patent Publication 2005/0231730, each expressly incorporated by reference herein.
A Fabry-Perot fiber optic sensor, shown in FIG. 1, is generally known in the art. A fiber optic Fabry-Perot sensor is an interferometric sensor. Light passes through optical fiber 10. The fiber 10 terminates at partially reflective surface 12a, which is itself aligned with partially reflective surface 12b. Surfaces 12a and 12b are separated by an air gap G which changes due to vibrations or other movement of at least one of the surfaces 12a, 12b. Preferably, surface 12a is fixed while surface 12b is affixed to the object being monitored and may therefore move so as to change the length of the gap G. For example, surface 12b may be affixed to diaphragms, other fibers, cantilever beams or other such structures in order to monitor the aforementioned parameters.
In operation, light travels through fiber 10 and some of this light is reflected back into fiber 10 by surface 12a. Additional light is also reflected back into fiber 10 when it strikes surface 12b. The light reflected from the two surfaces (i.e., that which is transmitted back into fiber 10 via surfaces 12a and 12b) interferes to create an interference pattern, also called a modulation pattern. When the interference pattern is monitored over time for changes, these changes are indicative of changes in the length of the gap G and very small changes or oscillations may be detected with this type of sensor.
Such Fabry-Perot sensors must be used in conjunction with detection and processing equipment in order to provide quantitative feedback concerning changes in gap G over a given period of time. Notably, these arrangements provide for absolute or static measurements as well as relative or dynamic measurements of the oscillations or vibrations caused by changes in the length of the gap G.
In the linear array signal processor (LASP), a cross-correlation pattern is produced when a wedge or Fizeau interferometer is placed in series with a Fabry-Perot sensor (interferometer). The correlation pattern is read out by a linear array of photodetector elements also referred to as pixels. The light source is, for example, a “white light” lamp with a wide output spectrum and the linear array of photodetector elements is, for example, a charge-coupled-device (CCD) made from silicon or InGaAs. Typically, optical fiber runs from the Fabry-Perot sensor to the signal conditioner which is connected to the interferometric correlation element. U.S. Pat. Nos. 5,202,939 and 5,392,117 issued to Bellville et. al. provide a description of a wedge and CCD device and are each also expressly incorporated by reference herein
An optical cross-correlation pattern (burst) is shown in FIG. 2. The pattern displays the correlation signal as a function of sensor gap. A common simplistic algorithm in the prior art looks for a feature in the burst such as the feature representing “the largest magnitude peak” or “the largest magnitude valley”, and uses this feature to determine the interferometric gap that the feature represents. This simplistic algorithm is not robust enough to be used when the Fabry-Perot gap and the wedge interferometer gap are made of different materials, e.g. air in the Fabry-Perot and transparent oxide in the wedge interferometer. The differences in the refractive index of the gap materials cause the shape of the burst to evolve as the gap in the Fabry-Perot interferometer changes as shown in FIG. 3, Graphs 1-9. This evolution in the shape of the burst can lead to serious errors in the accuracy of the measurement of the gap unless a more sophisticated signal-processing algorithm is used.
Other algorithms use a statistical method called “statistical correlation” to analyze the entire correlation burst and compare it to reference bursts. Such methods work to reduce noise but in the case of the evolving burst waveform, many reference burst signals are needed to provide reliable gap measurements. A description of one such algorithm is defined in US Patent Publication 2005/0073690 herein incorporated by reference.
Another simplistic algorithm analyzes the entire burst and computes an effective center-of-mass of the entire waveform. This method is not subject to discontinuous jumps but limits the dynamic range of the system and has poor resolution compared to the present invention.